Boats, cars, and other machines commonly include engines for the combustion of fuel and engine exhaust systems for exhausting the resulting combustion gas (also referred to herein as exhaust gas). A typical exhaust system will include a duct having an upstream end communicating with the engine and a downstream end communicating with an exhaust vent. The exhaust vent allows the combustion gas to be discharged and dispersed into an environment external of the engine.
One example of a problem that has occurred in marine engines is premature wearing of the engine and even "freezing up" of the engine. For example, an engine that has had relatively little use may be "frozen" and not work following a relatively short time since the last successful use, say even as short as one week. As is described further below, this problem appears now to be due to corrosion at the exhaust valves in the engine and or in other parts of the engine due to moisture that undesirably enters the engine. It now has been found that moisture droplets creep upstream in the engine exhaust duct even during engine operation and can enter the engine and cause damage. In fact, it has been found that especially in high performance engines the pressure wave occurring in the exhaust system may reflect, as wave functions can do particularly under appropriate tuned conditions, to carry moisture upstream along the duct walls, thus further exacerbating the moisture problem particularly in a marine engine where there are multiple sources of such moisture (that generated by the combustion process, that due to cooling water about and/or entering the exhaust duct, and/or that in which the boat is floating). The present invention is useful to overcome such problems, as will be described in greater detail below.
Also, marine engines typically route exhaust gases vertically upward a distance to form a "trap" above the water and to prevent sea water from entering the engine, unlike automotive applications which exhaust downward. This vertical upward orientation allows condensation to fall by gravity or to be pulled by exhaust pulsing back into the engine. The invention is useful to solve engine problems due to such moisture, too.
The engine combustion gas will usually include water products which commonly show themselves as moisture droplets in the exhaust duct. At least minor entry of water products or moisture droplets into an engine is believed to occur each time it is turned off. The water products entering the engine may warp exhaust valves, freeze rings, and produce other problems which generally corrode and reduce the life of the engine. In fact, entry of water products into engines of marine machines is believed to be a major cause of wear and premature mortality of marine engines. The severity of the problems caused by moisture droplets depends on many factors including exhaust pipe temperature, cam overlap, time at idle speed and lapse between engine uses (soak time). The more common use of high lift overlap cams in recent years has accentuated the problems caused by entry of the moisture droplets into the engine.
Many factors may contribute to the formation of moisture droplets in an engine exhaust system. For instance, combustion gas traveling in an exhaust duct usually has a flow profile which is parabolic in shape, the flow of the combustion gas near the interior, surface of the duct being slower than at the center of the duct. The moisture droplets tend to accumulate along the interior surface of the exhaust duct due to this flow profile. Additionally, moisture in the air used in the combustion of fuel may also contribute to the formation of water products, or moisture droplets. Still further, some engines include jackets which circulate water for cooling the engine itself and/or parts of the exhaust system. The cooling water exits the jacket downstream of the engine where it may be introduced into the exhaust system for mixing with the exhaust gases and subsequent discharge. Since the exhaust gases and/or exhaust duct may be too cool at that point of water introduction, such water may not turn to steam that would easily flow out with the combustion gases and therefore may be a source of water that enters to the engine.
It now has been found that some of the moisture occurring in an exhaust duct may creep or flow back upstream in the exhaust system against the flow of the combustion gases. Such upstream flowing moisture now has been found to be the source of corrosion problems in the area of the engine itself.
To minimize the entry of water products into engines, for example, those used in cars, trucks, etc., the downstream end of the exhaust duct may be positioned "lower" than the engine and upstream end of the exhaust duct. For example in cars and similar vehicles, the engine usually is mounted at a relatively high location for clearance above the road surface. The downstream end of the exhaust duct (i.e , the tail pipe) is usually located below the upstream end of the duct. Thus, gravity encourages the moisture droplets to flow towards the downstream end of the exhaust duct and exit from the engine exhaust system.
The positioning of the downstream end of the exhaust duct "lower" than the upstream end, as is common in cars, is not typical in some machines. For example, in marine machines such as boats, this positioning is usually not preferred. Instead, a boat engine (also referred to as marine engine) is usually mounted relatively low, usually below the water line of the boat, to maintain a low center of gravity. The exhaust duct often is mounted so that its downstream end is above the water line when the boat is moving under power whereby combustion gas will vent to the air rather than the water. Such a mounting arrangement will result in the downstream end of the exhaust duct being at a higher vertical position than the engine and the upstream end of the duct.
When the downstream end of the exhaust duct is higher than the upstream end, the combustion gas must flow upwardly when exiting the engine exhaust system. In this situation, gravity may encourage the moisture droplets to flow towards the upstream end of the exhaust duct and fall into the engine. Specifically, when the engine is turned off (and the flow of combustion gas ceases), gravity may pull the moisture droplets in the exhaust duct into the engine. Additionally, at idle and slow speeds, water condensation which forms on the cooled exhaust duct surface and that and/or other water in the exhaust system may travel into the engine through the exhaust valves. Still further and as was mentioned above, moisture droplets sometimes experience "back flow" in the exhaust duct, i.e., a tendency to creep upstream towards the engine even while the engine is operating, especially with engines having high overlap cams.
Thus, there is a need to reduce wear and/or damage to an engine due to water products in the engine exhaust system.